MAGIC FIND (Quantity / Rarity). What you need to know

The game needs some way of telling you "No, don't do that. It won't work." in regards to skill/support gem combos. It may be as simple as not letting you add a support gem to a linked skill if they are incompatible.

HAVE THE LINK LIGHT UP WHEN A SKILL AND SUPPORT GEM BECOME ACTIVE ONCE SOCKETED... IF IT DOESNT LIGHT UP...IT IS NOT COMPATABLE (JUST LIKE THE SKILL MAP LIGHTS UP)

It already does that. When a compatible support is linked to a compatible skill, a letter icon will show up on the skill hotbar.

The problem here as everyone saying is that MF is compatible, but only apply to the impact dmg, not the DoT afterward

The game needs some way of telling you "No, don't do that. It won't work." in regards to skill/support gem combos. It may be as simple as not letting you add a support gem to a linked skill if they are incompatible.

HAVE THE LINK LIGHT UP WHEN A SKILL AND SUPPORT GEM BECOME ACTIVE ONCE SOCKETED... IF IT DOESNT LIGHT UP...IT IS NOT COMPATABLE (JUST LIKE THE SKILL MAP LIGHTS UP)

I think this is a good idea. I know it already appears on the hotbar icon, but particularly for people wondering how support gems work, this would make it clear, for example, that the gems do not need to be immediately adjacent to each other to work.

I) For cases where death is caused by degen, and degen is aggregated into a single value and the per-application dps value is lost, then:

Ia) Average all MF/MR/etc across all effects.

Ib) Pick one of the applications (oldest, newest, random).

Ic) Re-implement to not lose application dps data, choose from II).

II) For cases where death is caused by degen where application values are maintained (not totals done, just potential) then:

IIa) Calculate (not simple average) weighted MF/MR from all effects based upon value (a 200hp/s degen counts twice as much as a 100hp/s, for purposes of application of MR/etc).

IIb) Using application time (new or old) as the order determinant, calculate which Degen was responsible for death - start with the oldest and see which decrement would have set the npc HP to 0 - use the values of that attack.

IIc) Do what was said previously, which is use the first application - this biases towards ranged degen viable since they'll tend to apply earlier than melee.

III) Keep a list of all damage associated with a mob for its lifetime (or a set period) and then on death iterate through the list and calculate values for mf/mr/etc based on actual damage done - this means that degen affects (MU* term) must then have accumulators (or timestamps and understand curtailed durations such as death/refresh). Exactly identical effects can be handled by an incrementer to avoid thousand entry long damage lists when there are only usually 5-10 possibilities for damage by a given pc (including curtailed degen/dots due to refresh/death).

This way seems at odds with the on-kill mechanics and FFA loot-explosion, but since there will be alternatives to strict FFA, loot system mechanics will need to be taken into account as they significantly change the 'fairness' of a particular implementation.

Sort-of IV: Create DoT support gems - different than on hit variants. For things like poison arrow, players would then have to choose which type to use. This would tend to make such gems 'half' as effective, though - which has to be balanced for and adds complexity.

Suffice to say that DoTs are the bane of programmers everywhere. I used MF/MR above, but the above systems can also apply (often two different ones at once) for + on kill effects - but generally overloading the two doesn't work since they tend to have different balancing requirements.

The game needs some way of telling you "No, don't do that. It won't work." in regards to skill/support gem combos. It may be as simple as not letting you add a support gem to a linked skill if they are incompatible.

HAVE THE LINK LIGHT UP WHEN A SKILL AND SUPPORT GEM BECOME ACTIVE ONCE SOCKETED... IF IT DOESNT LIGHT UP...IT IS NOT COMPATABLE (JUST LIKE THE SKILL MAP LIGHTS UP)

If a support gem is compatible with an active skill, it will apply it's cost multiplier to the cost, and you will see that support's letter appear on the skill icon. This will not occur if that support can't affect that active skill.

So let me get this straight. My Ranger with Poison Arrow build I been running around with for 55 levels had gained me no MF because its a Degen damage? Explains why i never get crap to drop.

From my experience... YES! RF and QF from gear does not count in the cloud damage of poison arrows

It does not. Now stop using poison arrow.

Yes it does. IIR and IIQ on gear affect all monsters you kill, including from degen. IIR/IIQ supports modify the damage dealt when hitting with the supporting skill such that if it kills the monster, it will apply the extra drop bonuses, but can't apply to other parts of the skill, only hitting for damage.